1. Field of the Invention
This invention relates to a solid state device for monitoring integral values of time and temperature of storage of perishables.
2. Brief Description of Prior Art
Perishable products such as fresh, refrigerated, and frozen, vegetables, fruits, meats, fish, poultry, dairy products, bakery products, juices, pre-cooked foods, soft and alcoholic beverages, and also including non-food items such as pharmaceuticals, vaccines, sera, blood, blood plasma, cosmetics, reactive chemical compounds, biochemical products, batteries, x-ray film and photographic films have measurable shelf-lives ranging from a few hours to some years, which are usually expressed within specified limits as the time left for available end use. The chemical kinetics of such reactions, including deterioration of perishables and devices to monitor this deterioration, can be determined with the Arrhenius equation: EQU In k=A-E.sub.act /RT
where,
In k=natural logarithm of the reaction rate PA1 A=pre-exponent constant PA1 E.sub.act =activation energy PA1 R=universal gas constant PA1 T=absolute temperature in degrees Kelvin.
In order to be an effective device for monitoring the shelf life of a perishable, parameters such as the activation energy, rate, and order of deterioration of the perishable should be substantially identical to those of the device monitoring its thermal degradation.
A large number of time-temperature monitor (TTM) devices for monitoring thermal degradation of perishables have been reported in the patent literature. Many of these devices are based on diffusion of a chemical from one matrix to the other, sometimes through a barrier, to introduce color change in the indicator in the other matrix. For example, in U.S. Pat. Nos. 4,195,056 and 4,195,058, G. N. Patel describes a device based on diffusion of vapor through a barrier film to introduce color change in the indicator on the other side of the barrier. Giezen et al in U.S. Pat. No. 4,154,107 describe a device which utilizes an acid in a pressure sensitive adhesive which migrates to contact an organic dye producing an aqueous-mediated color change. Kydonieus et al in U.S. Pat. No. 4,212,153 describe a device in which a dye preferably migrates from a lower plastisol layer to an upper indicator layer. The device can also utilize a barrier film to introduce an induction period to color change. Bradley et al in U.S. Pat. Nos. 4,292,916 describe a tape device which involves the migration of a dye from a carrier layer to a transfer layer to highlight a message. U.S. Pat. Nos. 3,520,124 to Myers describes a device to indicate a predetermined time interval based on two or more materials which react, either chemically or physically over a predetermined period to produce a termination signal. The reacting materials are carried on a base member and are separated by a barrier preventing contact. Upon elimination of the barrier, a commencement signal is produced indicating the time reaction is underway.
Other patents in the art include: U.S. Pat. Nos. 3,677,088; 3,967,579; 3,360,338; 4,057,029; 3,065,083; 4,188,437; 2,889,799; 3,078,182; 3,311,084; 3,386,807; 4,154,107. Most of these devices require an activator, a compound which induces color change in the indicator, and an indicator which needs to be kept separate prior to use. None of the above patents describe a device in which, the operative reactant and indicator are present in one solid matrix/layer and undergo a color change with time and temperature.
In most TTM devices, especially those based on diffusion of chemicals, the color changing reaction occurs between an indicator (a dye) and an active compound (activator). This reaction is fast. However, in U.S. Pat. No. 3,977,945 (to S. Tornmarck) a device is described where the active compound is produced by a slow reaction between two chemicals, e.g. an enzyme reacting with a protein to produce a carboxylic acid which reacts with a pH dye sensitive to induce a color change. However, these types of reactions are carried out in solution. In the above example, two solutions, a solution of enzyme in one envelope and a solution of substrate and indicator in another envelope, are kept separate prior to use. The device is activated by breaking the barrier between the envelopes and mixing the solutions.
In U.S. Pat. No. 3,999,946, Patel et al describe a single layer TTM device based on the solid state polymerization of diacetylenes. Though, the rate of reaction can be varied by co-crystallization (U.S. Pat. No. 4,238,352), the activation energy of polymerization can not be varied significantly. Diacetylenes are expensive specialty compounds and they polymerize via first order reaction to blue or red colors only.
Further, radiation dosimeters are described in U.S. Pat. Nos. 3,899,677, 4,001,587, 3,743,846, and 3,461,288. Generally these dosimeters contain an acid sensitive pH dye dispersed in a chlorine containing polymer, e.g. polyvinylchloride, optionally a plasticizer, wherein the polymer releases HCl upon contacting with ionizing radiation and causes a color change in the dye. The color change is proportional to the amount of ionizing radiation and thus the device serves as a radiation dosimeter. However, the device is not described as being able to function in the absence of contacting ionizing radiation as a time-temperature history indicator.
Research is constantly being conducted for producing a TTM device in which all reactants and indicators for producing a color change are combined in a single solid matrix layer, are not expensive specialty chemicals, and which can react to produce a color change in the absence of contacting with ionizing radiation.